Fault tolerance presents one of the greatest challenges to the design of modern high speed fiber optic networks. Unlike earlier point-to-point fiber optic subsystems, such as ESCON, for connecting computer systems, the modern fiber optic networks include interfaces which can handle a variety of different classes of transfers, for example, as defined for the "Fibre Channel" fiber optic network: Class 1--Dedicated connection (point-to-point); Class 2--Frame multiples (shared but guaranteed); Class 3--Datagram (ship-and-pray); and Class 4--Lossy voice and video. The characteristics of the interfaces are implemented in standard protocols, and will support many different transfer mechanisms, such as modern ESCON, FC, SSA, ATM, and FDDI protocols, to name a few. These protocols all employ a similar serial transport layer, comprising a "Fiber Optic Subassembly" (FOSA), fiber, and encoding/decoding modules. They are designed to be fast, dependable, and cover great distances, linking computer systems and components that are in different rooms, buildings and cities.
Demands on such links and networks have required that data availability and integrity be an important part of any network component design. As components have become more reliable, redundant, and fault tolerant, the interfaces linking these components or systems are now typically the most failure-prone aspect.
Hence, in order to provide error detection capabilities, most fiber optic protocols incorporate the IBM 8 to 10 bit encoding scheme with running disparity. This arrangement is described in the IBM Journal of Research and Development, volume 27, number 5, Sep. 1983, pp. 440-450, A. X. Widmer and P. A. Franazek, "A DC-Balanced, Partition-Block, 8B/10B Transmission Code" (herein, the Widmer and Franazek publication). There are also other checks built in to these protocols, providing several layers of protection. Many of the checks are logic circuits to detect that the frames are complete and that the protocols are correct.
Additionally, circuits are provided for assuring a level of error detection. For example, U.S. Pat. No. 5,229,875, Glista, provides a fiber optic coupler-repeater which analyzes the power of the optical signals from upstream terminals and compares the signal power to a preset value range and rejects those out of the range, activating a built-in "test circuit" alarm telling downstream terminals of the failure. As another example, U.S. Pat. No. 5,396,357, Goosen et al., describes a cross-channel data link which includes wavelength division multiplexing in opposite directions and logic circuity that "guarantees" that the interconnected systems have identical data irrespective of any single point failure in the data link.